Rock Mechanics and Subsurface Imaging at DUSEL, Homestake Mine

Rock Engineering 2009 - Rock Engineering in Difficult Conditions
Mario Magliocco, Steven D. Glaser, William M. Roggenthen,
Abstract The Deep Underground Science and Engineering Laboratory (DUSEL) is a scientific facility to be placed in the deeper reaches of the Homestake Mine, Lead, SD. Our team is carrying out the first scientific project awarded to the site – Towards a Transparent Earth. Taking advantage of the thousands of open exploratory and sand-line holes, we are installing a seismic imaging system. Each station has a 3-D accelerometer package and a 2 nr 2-D tilt meter.
In order to allow a wide variety of monitoring and characterization studies, the array instruments are sensitive over the frequency bandwidth of 1 Hz to 1 KHz. We have three three-component accelerometer stations currently installed – in a 140 m deep bore beneath the slurry pump house, and in 12 m deep bores on the 2000-foot level, 800 and 400 m from the Ross shaft. There is a tilt-meter installed at the 2000 level stations. These installations are in 150 mm diameter sand-holes. Current instrumentation is 100 mm in diameter and uses Wilcoxon model 731-207 accelerometers.
Besides a few dozen large diameter open holes available, there are more than 12,000 small diameter (~ 65 mm) exploration holes throughout the mine. We are assembling down-hole sondes based on an Altera FPGA. The current prototype has 24 input channels, both analog and digital, with a virtual real-time machine for each. Within the FPGA there are also real-time machines for the real-time clock, bus handling, and numerous control loops. All memory functions are handled seamlessly within the FPGA. Because the heart of the sonde is actually software on the FPGA, there is little need for physical hardware replacement for upgrades; a completely new set of machines can be implemented by installing new software over the web. The current system allows for a string of 28 sondes in a single bore hole. Local base stations for each string is just another sonde, which will in turn be serviced by a fiber optic Ethernet link so that the IEEE1588 protocol can be used to insure accurate timing. The system includes options to act as a wireless sensor network and GPS timing. Our preliminary research shows that accelerometers with a noise floor of 5 ?gvHz and a 3 dB bandwidth of 1 kHz can be fabricated in a form-factor that will result in a sonde at just 25 mm O.D. Each sonde will cost approximately $1,500. Each base station will cost approximately $1,000.
From the currently installed tilt sensors we are investigating solid earth deformation as a response to tidal potential. The accelerometers are being used to locate and characterize events associated with the mine dewatering. The preliminary results of these analyses will be available by January and reported in our paper.
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